Depth sounders which project sonic pulses from the hull of a vessel through the water to the bottom and then back again provide an indication of the depth of the water under the vessel. As is the case in all depth sounders, the round trip travel time of the sonic pulses projected into the water is measured, with the round trip travel time providing an indication of depth of reflecting targets, be it the bottom of the body of water or some other target.
As to digital depth sounders, were the water involved to be clear of refuse, mud globules, thermals, or small targets such as bait fish, bubbles, non-laminar flow or turbulence produced around the hull, the digital depth reading would be stable. However, this is not the case. What in fact occurs is that the above conditions cause the digital depth sounder either to read the shallowest depth for which the depth sounder is designed, or to cause the depth indication to flip around, sometimes in a mindless fashion, with the returns seeming to come from both shallower depths than the bottom, as well as much deeper depths as the depth sounder attempts to lock onto the bottom in a relock cycle.
In short, depth sounders give erratic readings when the water quality is degraded by non-miscible materials such as refuse, mud or physical objects in the water, is replete with small fish in layers beneath the vessel, is inundated with oil or sludge which reduces transducer efficiency, or is disturbed by bubbles or turbulence. These conditions are grouped together and referred to herein as "dirty" water. Since the depth sounder usually reads its shallowest possible depth in these circumstances, this is not only annoying; but, in some cases, safety is compromised.
The problem is not dissimilar to close in clutter in radar systems in which time varying gain, properly adjusted, adjusts the gain of the receiver to such a low level that near in clutter is not detected by the receiver. The time varying gain of a system involves the gain adjustment from zero along a T.sup.3 or T.sup.4 curve in that the receiver gain is opened in a non-linear fashion from the beginning of the transmitted pulse.
While time-varying gain circuits are available to eliminate the effect of certain dirty water conditions, they are ineffective especially in Boston Harbor and in newly dredged channels because the amount of particulate matter in the water prevents the lock up of depth sounders to the bottom due to the density of the churned up material or because oil or sludge coats the face of the transducer which drastically reduces its response. Dirty water is most evident after storms or in newly dredged channels, or indeed where actual refuse or oil is dumped into the body of water. The relock process for most depth sounders can be confused to such an extent by the small dispersed targets that the microprocessor-driven algorithms cannot provide appropriate lock to the bottom. Erratic or low readings are also caused by opening the gain of the receiver all the way in the presence of such dirty water. The net result is that the physical phenomenon of dirty or unsuitable water prevents accurate readings from being taken, with the depth sounder locking up to its shallowest depth.